Spark-ignition engines are provided with crankcase-ventilation systems. The “blow-by” gases, which are mixed with engine oil and flow from the combustion chamber past the piston of a cylinder into the crankcase, are first conducted to a liquid-vapor separator and then returned to the intake system of the engine for combustion, this return taking place at a suitable location where negative pressure is ideally present in nearly all operating states of the engine. To limit the negative pressure in the crankcase, a calibrated throttle valve is integrated in the ventilation duct. In turbo-engines, it is distinguished between partial load and full load ventilation. The full-load ventilation is initiated downstream from an air filter. The partial-load ventilation is conducted to the intake manifold. The result of a defect in the crankcase-ventilation system is, first of all, the escape of harmful HC emissions into the environment and, secondly, a changed operating point of the engine, especially during idling. When intake-manifold pressure sensors are used, such a defect will not lead to a mixture deviation since the measured intake-manifold pressure is always correct and it is basically unimportant from where the combustion air comes. If an air-mass flow sensor is used for detecting the load, a lasting mixture deviation would be produced and diagnosed.